In a scanning printer which produces continuous tone images, an exposing device in the printer should travel across an image recording medium with as near a constant scanning velocity as possible in order to produce a high quality image on the image recording medium. Presently, circuitry which controls the positioning of the exposing device cannot meet this requirement to produce a sufficiently constant scanning velocity for the exposing device relative to the image recording medium. Accordingly, data circuitry which controls both the timing and the image management aspects of the printer compensates for this inability by using a variety of different approaches.
One approach that has been used extensively is to employ a variable frequency oscillator as a high frequency pixel clock and a phase locked feedback loop interconnecting the high frequency clock to a position encoder. Critical to the success of this approach is the design of the feedback loop. If the loop responds too slowly, the position of pixels on the image recording medium will wander excessively with small changes in the scanning velocity of the exposing device. On the other hand, if the loop responds too fast, the phase locked feedback loop will tend to ring in response to sudden changes in the scanning velocity of the exposing device. Regardless of the tuning of the loop, this approach exacerbates errors in print density that arise when the scanning velocity changes. When the scanning velocity of the exposing device increases, the energy density of the exposure necessarily decreases which results in underexposure of the image recording medium. In addition, the variable frequency phase locked loop increases its frequency which shortens the exposing time for the exposing device. For constant exposing power, this means that less total energy is delivered to the image recording medium which, in turn, results in further underexposure of the image recording medium.
To avoid the problems associated with the feedback loop system, another approach has been employed. This approach employs: a spatial clock which produces a spatial clock pulsetrain wherein each spatial clock pulse defines the amount of time required for an exposing device to travel across a set distance on the image recording medium; a fixed timing clock which produces timing clock pulses at a rate substantially faster than the spatial clock; and a counter which counts the timing clock pulses until a terminal value is reached and which has a phase that is periodically shifted to match that of the spatial clock. In this approach, the rising edge of the spatial clock triggers the counter to start counting to a terminal value. When the value of the count reaches a value associated with a desired print density of the pixel to be printed, the intensity level of the exposing device is switched from a first state which is the "on" state to a second state which is the "off" state. Then, when the counter reaches the terminal value, the intensity level of the exposing device is again switched from the second state back to the first state.
This approach provides the proper duration of the intensity level of the exposing device at the proper state when the spatial clock is running at the normal rate. However, when the spatial clock is substantially faster than normal or substantially slower than normal, problems occur. This approach may keep the intensity level of the exposing device at a given state for too long a period when the spatial clock is substantially slower than normal. This occurs because the count reaches the terminal value before the next rising edge of the next spatial clock pulse and thereby switches the intensity level of the exposing device to the first state too soon. This approach may also prevent the printing apparatus from switching the intensity level of the exposing device to the first state when the spatial clock is substantially faster than normal. This occurs because the count never reaches the terminal value so that the intensity level of the exposing device can change from the second state to the first state.
As a result, a need still exists in the art for improved data circuitry that can control the intensity level of the exposing device so that the desired pixel density can be consistently disposed on the image recording medium in a manner whereby the deleterious effects associated with a fast or a slow spatial clock are minimized.